Preface

Abstract

Smart structures research in the 1980s was motivated strongly by applications in space. Space structures must typically be light, stiff and dimensionally precise, while operating in an extreme thermal and radiation environment. Embedding sensors and actuators to control structural behavior with active vibration damping or isolation was a major thrust. Large strain materials such as shape memory metals were felt to be useful for structural shape control and for large motion deployments. Another research thrust was in the area of in situ fiber optic sensors for measurement of strain and vibration, and for structural health monitoring. As we approach a new millennium, it is useful to see the progress that has been made in space applications and to speculate on areas which might bear fruit in the future. This special issue of Smart Materials and Structures reports on progress in active vibration damping and isolation, along with three space flight experiments to verify these new technologies. We are encouraged to see significant progress in France and Germany, in addition to the US. Research is motivated by missions such as the OISI space interferometer, which uses active members to damp structural motion, and active optical delay lines with piezoceramic fine control actuators to compensate for residual structural motion. Applications such as powerful lasers in space will require large active members such as those in ASTREX to quickly settle out large vibrations. There are three papers on active isolation. The SEPTRA active hardmount employs piezoceramic stack actuators. Two active softmounts employing magnetic actuators are described in this issue. These are the VISS isolator and ARIS, the exquisite isolator slated to enable micro-gravity research in the Space Station. Tremendous progress has been made in shape memory metals (SMM) for non-explosive deployment devices; numerous types have now flown in space. We were not able to obtain a specific paper on SMM for this issue, but the inclusion of two such devices, one for deployment and one for clamping, in the ACTEX experiment is not atypical. Space robotics is a very pertinent field, with many spacecraft having numerous appendages that must be controlled precisely. We were fortunate to receive two excellent papers from Japan in this area. One deals with a very new field, the adaptive truss. The other proposes a new concept, an appendage with redundant degrees of freedom allowing reactionless motion within the workspace. The field of fiber optic sensors continues to move ahead, with the Bragg grating approach offering an opportunity to multiplex numerous sensors on a single fiber. Perhaps the greatest advantage of FOS for space is the ability to minimize wiring harnesses. A hurdle still to overcome is the temperature effect on index of refraction. As we look to the future, structures spanning hundreds of meters are envisioned. Inflatable membranes is one approach, and tensegrity structures is another. Producing these gossamer structures with micro-precision is a challenge where smart structures will surely play a role. One message from this field is that one should look for the technology that is best for a given structural problem, not try to force fit smart structures. In special cases, the smart structure will be the solution. Indeed, the fix for the Hubble Space Telescope's blurry vision was a smart material to actively position a corrective optic. In closing we would like to thank the authors for their effort and patience as this issue came together. Especially we thank Vijay Varadan, our Editor-in-Chief, for his constant support, and Annette Lose, his special assistant, for a marvellous job in organizing the papers and review process. Guest Editors Allen J Bronowicki TRW Space and Electronics Group Alok Das Air Force Research Laboratory Space Vehicles Directorate Ben K Wada Jet Propulsion Laboratory